Loudspeaker Ports
Ken Lofthouse
6792&3). After a while I noticed that a terminal was cracked and after
a futile attempt of procrastinating the inevitable (using duct tape!) I
contacted Spendor and obtained a pair of replacement updated (gold
plated ) terminals and fitted them. When I had taken the back off I
found that in fact it was a Bass Reflex design but no ports were fitted.
I then emailed the tech line of Spendor whose comment was "perhaps they
didn't think of it but you can fit ports if you want to". This response
could best be described as underwhelming!
Has anyone had any experience with doing this as I have no issues with
the speakers as they are. Fitting ports would in effect increase the
path length to the port and thus alter the frequency at which the port
acts. Were later units fitted with ports?
I apologise if this topic has been "flogged to death" in the past but I
have only been passively monitoring this ng for a couple of months.
Thanks
Ken
Stewart Pinkerton
wrote:
Sorry, I'm completely confused here. The BC1 is and always was a bass
reflex design. Whoever blocked off the ports, it wasn't Spendor!
Alernatively, exactly what do you mean by 'ports'?
--
Stewart Pinkerton | Music is Art - Audio is Engineering
Richard D Pierce
Ken Lofthouse <peri...@netcom.ca> wrote:
>In June 2000 I bought a pair of speakers on ebay (Spendor BC1 - S/N
>6792&3). After a while I noticed that a terminal was cracked and after
>a futile attempt of procrastinating the inevitable (using duct tape!) I
>contacted Spendor and obtained a pair of replacement updated (gold
>plated ) terminals and fitted them. When I had taken the back off I
>found that in fact it was a Bass Reflex design but no ports were fitted.
If it's a bass reflex design, there IS a port fitted, by
definition. It may not have a "tube," but that's okay, as you
will leanr below.
>I then emailed the tech line of Spendor whose comment was "perhaps they
>didn't think of it but you can fit ports if you want to". This response
>could best be described as underwhelming!
This is a completely lame response from someone who is clueless.
>Has anyone had any experience with doing this as I have no issues with
>the speakers as they are.
Then why worry about it?
>Fitting ports would in effect increase the
>path length to the port and thus alter the frequency at which the port
>acts.
The "path length to the port" is not what determines the
frequency at which the port acts. That frequency is determined
by the volume of the enclosure and the length and diameter of
the port or vent.
>Were later units fitted with ports?
>I apologise if this topic has been "flogged to death" in the past but I
>have only been passively monitoring this ng for a couple of months.
For a vented ("bass reflex", "ported") speaker to work, it has
to have a secondary aperture from the enclosure to the outside.
That aperture can have a tube in it, as you seem to expect, or
it can simply be anaperture: the exact configuration is
dependent upon the design of the system.
To understand what Spendor did and why (and people like Rogers
did the same thing in THEIR version, since they were copying a
BBC-licensed design), you need to understand why the port is
there to begin with.
The port, vent, tube, what have you, provides what's called an
"acoustical inertance" or, effectively, a mass. That mass, in
conjunction with the compliance of the air in the enclosure,
forms a second resonant system whch, without getting into the
details, gives the designer a couple of more degrees of freedom
in implementing the system. One thing it allows you to do is to
shoehorn a more efficient system into the same size box as a
closed system without sacrificing low frequency extension.
The greater the mass, all other things being equal, the lower
the frequency to which that resonant system is tuned. In this
context, lower is NOT better, rather, the right frequency is
what is aimed for, and that depends upon the driver's
parameters, the desired system response, etc..
The effective mass of the part is largely determined by the
length of the port divided by the square of the diameter of the
port. In addition, there is what is called the "end coprrection"
factor which effectively makes the port appear longer than it
really is.
For a given system design, then, the port must provide the right
amount of acoustic mass to tune the system properly. Given the
interelation between length and diameter and end correction,
there are a LOT of choices. If the designer though the port
diameter should be larger, then the length of the port MUST be
longer to get the same effective acoustic mass.
Now, remeber that end correction we talked about?That exists
because as the air is moving in and out of the port, it tends to
keep traveling in a straight line once it's moved from the end
of the port AS IF the port was still there, simply due to the
physical inertia of the air. So the port ACTS us if it's a
little longer. Depending upon whether the port end flush with a
baffle ("flanged") or hangs out in the middle of the air
("unflanged") the correction factor can be of the order of 0.42
to 0.3 times the diameter of the port, respectively
For a conventional port, say, 2" in diameter, with one end flush
with the baffle, the other free on the interior, the end
corrections would add 2*0.42 + 2*0.3 or almost 1 1/2" to the
effective length of the port.
In the case of the BBC designs, it was decided quite
deliberately, to use a smaller diameter port. One of the smaller
8" 2-way designs utilized a port with an effective diameter of
about 2", and was simply a hole cut in the front baffle. the
front baffle was 1/2" thick, added to the 1 1/2" end corrections
applied, results in an effective acoustical length of 2".
There are two ways, in fact, to tune and enclosure by way of
adjusting the port: pick a fixed diameter and vary the length,
or pick a fixed length and vary the diameter. The BBC chose to
do the latter, and withing the parameters they were working
with, it's a perfectly valid technical choice.
So, if you were to succumb to the desire to stick a tube in that
port, you'd likely screw the system up: There is method to their
apparent madness, even if the guy you talked to sound
certifiably mad.
--
| Dick Pierce |
| Professional Audio Development |
| 1-781/826-4953 Voice and FAX |
| DPi...@world.std.com |
Kalman Rubinson
wrote:
>Sorry, I'm completely confused here. The BC1 is and always was a bass
>reflex design. Whoever blocked off the ports, it wasn't Spendor!
>Alernatively, exactly what do you mean by 'ports'?
And, fwiw, the round port is on the front panel.
Kal
Richard D Pierce
Yes, but on the old Spendor and Rogers BBC Monitors, the only
way to get to the drivers was by taking the back off. The grill
was not removable. So if you weren't aware of the existance of a
port to begin with, you'd only learn of it by taking the back
off (all the drivers were rear-mounted, including the tweeter,
the old venerable Celestion HF-1300, which as designed to be
rear-mounted to begin with. It's flange was som 80 mm back from
the front, so that rear-mounting it resulted in it being flush
with the front of the baffle).
Stewart Pinkerton
Yes, and it seems that my suspicion was correct. The poster found a
box with holes in it, but expected tubes. As Dick rightly points out,
those holes still contain an effective air mass due to end effects.
Ken Lofthouse
Each BC1 has a circular "hole" in its front panel but there is neither a
circular nor rectangular tube attached to the "hole". You might thus describe
the ports as "zero length". They are not "blocked off" as in converting to IB
or AS.
Regards
Ken Lofthouse
Richard D Pierce
Ken Lofthouse <peri...@netcom.ca> wrote:
>Each BC1 has a circular "hole" in its front panel but there is neither a
>circular nor rectangular tube attached to the "hole". You might thus describe
>the ports as "zero length".
When, in fact, they are a port equal to the thickness of the
front panel (19 mm?) plus about 0.72 the effective diameter due
to the end correction factor.
Stewart Pinkerton
wrote:
>In article <a96ui...@enews3.newsguy.com>,
>Kalman Rubinson <k...@nyu.edu> wrote:
>>On Fri, 12 Apr 2002 07:32:46 GMT, a...@borealis.com (Stewart Pinkerton)
>>wrote:
>>
>>>Sorry, I'm completely confused here. The BC1 is and always was a bass
>>>reflex design. Whoever blocked off the ports, it wasn't Spendor!
>>>Alernatively, exactly what do you mean by 'ports'?
>>
>>And, fwiw, the round port is on the front panel.
>
>Yes, but on the old Spendor and Rogers BBC Monitors, the only
>way to get to the drivers was by taking the back off. The grill
>was not removable. So if you weren't aware of the existance of a
>port to begin with, you'd only learn of it by taking the back
>off (all the drivers were rear-mounted, including the tweeter,
>the old venerable Celestion HF-1300, which as designed to be
>rear-mounted to begin with. It's flange was som 80 mm back from
>the front, so that rear-mounting it resulted in it being flush
>with the front of the baffle).
And of course, that's a good way to date your BC1, since only the
first couple of years production lacked the Coles 'supertweeter'.
OTOH, all those early models are getting a bit tired now, since the
Coles driver was added around 1973.